1. Field of the Invention
The present invention relates to the field of photomultipliers, and in particular to a solid dynode structure which is rugged and exhibits good pulse height resolution.
2. Description of the Prior Art
Photomultipliers come in a wide variety of forms. One type of photomultiplier utilizes a so-called "venetian blind" dynode structure wherein the dynode is formed from a series of slats or vanes arranged at an angle to the path of a photoelectron emitted from the photoemissive surface of a photocathode, or a secondary electron emitted from a preceding dynode stage. Such dynode structures are well-known and are shown in U.S. Pat. No. 3,498,834 and described in the RCA Photomultiplier Handbook, 1980, page 29.
The venetian blind dynode structure is particularly useful in environments where the photomultiplier tube may be subject to large shocks or vibrations, such as might be encountered in a spacecraft or in a well-logging operation. This is because the dynode is fully supported about its periphery by an electrically conductive mounting which also acts as the external electrical connection for the dynode.
One drawback to the venetian blind dynode structure is that it does not provide good pulse height resolution for low levels of incident photoelectrons. Under very low light level conditions (e.g., 50 or less electrons incident on the dynode), the output of the photomultiplier tube will no longer be proportional to the number of electrons impinging on the first dynode, but will fluctuate in a random fashion due to statistical variations in the numbers of secondary electrons emitted by the dynodes and the statistics of capturing and guiding these secondary electrons to the next dynode. The ability to discriminate between events which produce different numbers of photoelectrons is also reduced because of these statistical fluctuations.
These variations are due, in part, to the open structure of the venetian blind dynode which allows a large percentage (e.g., 30%) of the incident primary electrons to completely miss the first dynode.
The collection of secondary electrons by a succeeding dynode stage also varies with the position the incident primary electron strikes the vane of the preceding dynode stage. This is because each venetian blind dynode usually includes a screen or grid arranged just ahead of it which is held at the same potential as its associated dynode. This sets up an area of equal potential near the forward (leading) edges of the vanes of the dynode, and serves to improve collection of the secondary electrons emitted from the dynode by allowing the electrons to travel a short distance from the dynode within the region of equal potential. However, secondary electrons emitted from the leading edge of a dynode vane are less likely to be accelerated to the next dynode stage than are secondary electrons emitted from the rearward (trailing) edge of the vane because the influence of the potential applied to the next dynode stage is slightly less on the leading edge of the vane than on the trailing edge. This effect makes the collection efficiency of a succeeding dynode stage somewhat dependent on the location of where a primary electron strikes the preceding dynode.
One alternative type of dynode structure which does provide relatively good pulse height resolution is the so-called "box and grid" dynode structure, such as shown on page 29 of the aforementioned RCA Photomultiplier Handbook. However, such a dynode structure has the drawback that the dynode and electron focusing elements cannot be easily mounted within the photomultiplier tube in a rugged manner. This prevents this type of dynode structure from being used in those environments where the tube may be subject to high levels of shock and/or vibration. Also, the components used in the box and grid dynode structure are not symmetrical and thus can result in asymmetries in electron focusing and collection unless their designs are carefully executed.
In order to improve the electron collection efficiency in a photomultiplier it is known to use a dynode formed from a solid sheet of material oriented in a plane substantially perpendicular to the path of electrons to be accelerated. Such solid dynode structures are shown in U.S. Pat. Nos. 2,196,278 and 2,203,225. The arrangement shown in U.S. Pat. No. 2,196,278 utilizes dynodes formed from a thin metal foil which are mounted to the wall of the photomultiplier by a single, slender support arm. However, this dynode structure is too weak to be able to withstand high levels of shock or vibration. Also the secondary electron coefficient of this device is low because an incoming primary electron must travel through the foil dynode before a secondary electron is emitted.
U.S. Pat. No. 2,203,225 shows a disk-type solid dynode arranged with its surface substantially perpendicular to the longitudinal axis of the photomultiplier tube and surrounded by a cup-shaped electron focusing structure. The dynode and cup-shaped focusing structure are mounted to the wall of the photomultiplier using a single support arm. This arrangement, similar to the support arm used in U.S. Pat. No. 2,196,278, will not support the dynode structure properly if the photomultiplier tube is subject to high levels of shock or vibration.
In addition, both these photomultiplier tubes require the use of external magnetic focusing coils in order to focus the electrons being accelerated by each stage of the photomultiplier tube. Such focusing coils increase the diameter and weight of the photomultiplier tube, thus rendering them less desirable for use in situations where size is critical, such as in the well-logging or spacecraft environments. Also, such focusing coils require a separate source of electrical potential, in addition to the potential source for the dynodes.